Neuromuscular disorders in the critically ill

Clinical presentation of neuromuscular respiratory failure

Patients experiencing respiratory dysfunction as a result of neuromuscular disease typically present with a combination of upper airway dysfunction and diminished tidal volume (V _T ). Upper airway muscle weakness typically presents with difficulty swallowing liquids, including respiratory secretions, along with a hoarse or nasal voice. In addition to the risk of aspiration, these patients have difficulty with negative-pressure ventilation because the weakened muscles cannot keep the airway open as the pressure falls.

Loss of V _T occurs secondary to weakness of the inspiratory muscles. In addition to diaphragmatic weakness, which may present with paradoxical abdominal movement, parasternal intercostal muscle weakness also causes diminished V _T by preventing the chest wall from expanding against negative intrapleural pressure. Thus lower cervical spinal cord injuries may induce respiratory failure despite preserved phrenic nerve function. The patient often attempts to maintain V _T by contraction of accessory muscles such as the sternomastoids. As parasternal intercostal muscles develop spasticity over weeks, respiratory function of this type typically improves and may permit weaning from mechanical ventilatory support, at least during the daytime.

Patients with progressive generalized weakness (e.g., Guillain-Barré syndrome) commonly begin to lose V _T before developing upper airway weakness. To maintain minute ventilation and carbon dioxide excretion, a patient’s respiratory rate increases. Respiratory rate is thus one of the most important clinical parameters to monitor. As the vital capacity falls from the norm of about 65 mL/kg to 30 mL/kg, a patient’s cough weakens, and clearing secretions becomes difficult. A further decrease in the vital capacity to 20 mL/kg to 25 mL/kg results in an impaired ability to sigh, resulting in progressive atelectasis. At this point, hypoxemia may be present because of ventilation-perfusion mismatching, and an increasing percentage of V _T is used to ventilate dead space. Respiratory failure is imminent, and ICU admission is recommended for all patients with a vital capacity <20 mL/kg. The precise point at which mechanical ventilation is necessary varies with the patient, the underlying condition, and especially the likelihood of a rapid response to treatment.

Regardless of the vital capacity, indications for intubation and mechanical ventilation include evidence of fatigue, hypoxemia despite supplemental oxygen administration, difficulty with secretions, and a rising arterial partial pressure of carbon dioxide (PaCO ₂ ). In the absence of hypercapnia, occasional patients (e.g., those with myasthenia gravis) can be managed with very close observation in an ICU with less invasive techniques (e.g., bilevel positive airway pressure [BiPAP]).

In addition to vital capacity, trended measurements of the maximum inspiratory pressure (PImax, also called negative inspiratory force [NIF]), are useful indicators of ventilatory capacity. The inability to maintain a PImax greater than 20–25 cm H ₂ O usually indicates a need for mechanical ventilation. Although the maximum expiratory pressure (PEmax) is a more sensitive indicator of weakness, it has not proven to be as useful as an indicator of the need for mechanical ventilation.

Because a patient with neuromuscular respiratory failure has an intact ventilatory drive, the fall in V _T is initially matched by an increase in respiratory rate, keeping the PaCO ₂ normal or low until the vital capacity becomes dangerously reduced. Many patients initially maintain their PaCO ₂ in the range of 35 mm Hg because of either (1) a subjective sense of dyspnea at low V _T or (2) hypoxia from atelectasis and increasing dead space. When the PaCO ₂ begins to rise in these circumstances, abrupt respiratory failure may be imminent, as CO ₂ displaces more oxygen from the alveolar gas. The modest degree of hypoxia in most of these patients therefore worsens when the PaCO ₂ begins to rise. Moreover, aspiration pneumonia and pulmonary embolism are also frequent causes of hypoxemia in these patients. To determine the relative contributions of these conditions to a patient’s hypoxemia, one can use a simplified version of the alveolar gas equation:

where P a O ₂ is the alveolar partial pressure of oxygen, PiO ₂ is the partial pressure of the inspired oxygen (in room air, 150 mm Hg), and R is the respiratory quotient (on most diets, about 0.8). This allows for the estimation of the alveolar-arterial oxygen difference (P a O ₂ − PaO ₂ ). Under ideal circumstances in young people breathing room air, this value is about 10 mm Hg, but it rises to about 100 mm Hg when the fraction of inspired oxygen (FiO ₂ ) is 1.0. The alveolar air equation allows one to assess the contribution of hypercarbia to the decrease in arterial partial pressure of oxygen (PaO ₂ ); it determines whether there is a cause of significant hypoxemia in addition to the displacement of oxygen by carbon dioxide.

Physicians must observe patients for rapid, shallow breathing ; recruitment of accessory muscles; and paradoxical movement of the abdomen during the respiratory cycle. Direct observation is particularly important, as patients may have orbicularis oris weakness, causing an artificially low vital capacity and NIF measurements because they cannot form a tight seal around the spirometer mouthpiece. In addition to physical examination findings, fluoroscopy of the diaphragm is occasionally valuable for the diagnosis of diaphragmatic dysfunction.

Autonomic dysfunction commonly accompanies some of the neuromuscular disorders requiring critical care, such as Guillain-Barré syndrome, botulism, and porphyria ( Table 49.1 ). In Guillain-Barré syndrome (vide infra), dysautonomia is common and may arise in parallel with weakness or may follow the onset of the motor disorder after one or more weeks.

Neuromuscular diseases precipitating critical illness